Transcript Wireless Web Performance

Performance and Robustness Testing
of Wireless Web Servers
Guangwei Bai
Kehinde Oladosu
Carey Williamson
November 26, 2002
TeleSim Research Group
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1. Introduction and Motivation
 Observation: the same wireless technology
that allows a Web client to be mobile also
allows Web servers to be mobile
 Idea: portable, short-lived, ad hoc networks
 Possible applications:
o classroom area networks, seminars
o press conferences, media events
o sporting events, gaming, exhibitions
o conferences and trade shows
o disaster recovery sites, field work, etc.
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Background: Portable Networks
 Assumptions: the characteristics of a
portable short-lived network are:
o set it up when needed; tear down after
o only needed for minutes or hours
o when may not be known a priori
o where may not be known a priori
o no existing infrastructure of any kind
o general Internet access not available
o general Internet access not required
o pre-defined content; target audience
o 1-100 users; mobile; limited bw needed
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2. Objectives
 to assess feasibility of portable networks
 to benchmark the performance capabilities
and limitations of an Apache Web server in
a wireless ad hoc network
 to identify the performance bottlenecks
 to understand impacts of different factors
o number of clients
o Web object size
o persistent connections
o transmit power (energy consumption)
o wireless channel conditions
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3. Experimental Setup
• Compaq Notebooks (1.2GHz Pentium III, 128MB RAM,
512 KB L2 cache, Cisco Aironet 350 network cards)
• RedHat Linux 7.3, httperf, Apache 1.3.23, SnifferPro 4.6
• Network: 11 Mbps IEEE 802.11b wireless LAN, ad hoc mode
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Experimental Setup (Cont’d)
• IEEE 802.11b: a standard for wireless LANs
Carrier Sense Multiple Access with Collision Avoidance
(CSMA/CA), up to 11 Mbps data rate at physical layer
• ad hoc mode
frames are addressed directly from sender to receiver
• httperf
Web benchmarking software tool developed at HP Labs
• Web server: Apache (version 1.3.23)
Process-based, flexible, powerful, HTTP/1.1-compliant
• SnifferPro 4.6
real-time capture, recording all wireless channel activity,
enabling protocol analysis at MAC, IP, TCP and HTTP layers
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4. Experimental Design
• Impacts of different factors on wireless Web server
performance (one-factor-at-a-time)
Experimental Factors and Levels
Factor
Number of Clients
HTTP Transaction Rate (per-client)
Levels
1, 2, 4
10, 20, 30, …, 160
HTTP Transfer Size (KB)
Persistent Connections
HTTP Requests per Connection
Transmit Power (mW)
1, 2, 4, 8, …, 100
no, yes
1, 5, 10, 15, …, 60
1, 5, 20, 30, 50, 100
Client-Server Distance (m)
1, 10, 100
• Performance metrics
– HTTP transaction rate, throughput, response time, error rate
at Application Layer,
– TCP connection duration at Network Layer
– Transmit queue behaviour at Link Layer,
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5. Measurement Results and Analyses
- Expt 1: Request Rate
- Expt 2: Transfer Size
- Expt 3: Number of Clients
- Expt 4: Persistent Connections
- Expt 5: Transmit Power
- Expt 6: Wireless Channel
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Experiment 1: Request Rate
Purpose: to determine the range of feasible and sustainable
loads for the wireless Web server
Design:
• Number of Clients: 1
• HTTP transaction rate: 10, 20, …, 160 req/sec
• HTTP transfer size: 1 KB (fixed)
• Persistent connections: no
• Transmit power: 100 mW
• Client-server distance: 1 meter (on same desk)
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Wireless Web Performance at Application Layer
Main observation:
• As the offered load increases:
linear increase  instability  lower plateau
• Peak throughput < 1 Mbps for 1 KB transfers
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Transmit Queue Behaviour for Experiment 1
Main observation: Wireless LAN is the bottleneck
• Packet drops occur from link-layer queue (client side)
• Even before they get on the wireless LAN!!!
Reason:
• No flow control / backpressure mechanism
• Note: default queue size is 100 in the Linux kernel
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Wireless Web Performance at Application Layer (Cont’d)
Main observation:
• the response time is about 9 ms at low load, increase
significantly to over 2 sec at high load (>85 req/sec)
• failures occur frequently under overload
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Measurement at Network Layer
Low load: 10 req/sec
Stable performance
Mean: 9.7ms
Medium load: 50 req/sec
Greater variation, 2 spikes
Mean: 10ms
High load: 80 req/sec
More variability,
some spikes, slight skew
Overload: 100 req/sec
Queue buildup,Packet drops,
Retransmissions,TCP resets
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Experiment 2: Transfer Size
Purpose: to study impact of HTTP response size
Design:
• Number of Clients: 1
• HTTP transaction rate: 10 req/sec (fixed)
• HTTP transfer size (KB): 1, 2, 4, 8, …
• Persistent connections: no
• Transmit power: 100 mW
• Client-server distance: 1 meter (on same desk)
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Measurement at Network Layer
General observation:
as HTTP transfer size
increases, mean TCP
connection duration
increases, as does the
variance of distribution.
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Measurement at Network Layer
Light load: 8 KB
Duration: 24 msec
Throughput: 2.8 Mbps
Medium load: 32 KB
Duration: 67 msec
Throughput: 3.9 Mbps
Overload: 64 KB
Duration: >100 msec
Throughput: 4.1 Mbps
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Experiment 3: Number of Clients
Purpose: to study impact of high load generated by
multiple clients
Design:
• Number of Clients: 2, 3, 4
• HTTP transaction rate: 10, 20, …, 160 req/sec
• HTTP transfer size: 1 KB (fixed)
• Persistent connections: no
• Transmit power: 100 mW
• Client-server distance: 1 meter (on same desk)
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Wireless Web Performance at Application Layer (4 Clients)
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Wireless Web Performance at Application Layer (4 Clients)
Main observation:
• 4 clients share network and server resources equally
• 30% higher aggregate throughput (110 conns/sec)
• bottleneck is now at server network card (drops!!)
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Wireless Web Performance at Application Layer (2 or 3 Clients)
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Wireless Web Performance at Application Layer (2 or 3 Clients)
Main observation: unfairness problem at high loads:
one client obtained a higher proportion of the
throughput at expense of another (don’t know why?)
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Experiment 4: Persistent Connections
Persistent Connections:
• Multiple HTTP transactions can be sent on the
same TCP connection.
• amortize overhead of TCP connection processing
• reduce memory consumption for TCP state
Purpose of this experiment: to study impact of
persistent connection on wireless Web performance
Design:
• Number of Clients: 1 and 2
• HTTP transaction rate: 10 req/sec (fixed)
• HTTP transfer size: 1 KB (fixed)
• Persistent connections: yes
• Transmit power: 100 mW
• Client-server distance: 1 meter (on same desk)
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Achieved Throughput for Experiment with Persistent Connections
Main observation:
• Peak throughput: 3.22 Mbps, 3.5x improvement
over non-persistent connections (0.9 Mbps),
• two clients share the server and network resources
equally
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Experiment 5: Transmit Power
Energy consumption- an important issue for mobile
Clients and Server.
Purpose: to see what transmit power is required for
acceptable performance in classroom setting
Design:
• Number of Clients: 1
• HTTP transaction rate: 10 req/sec (fixed)
• HTTP transfer size: 1 KB (fixed)
• Persistent connections: no
• Transmit power: 1, 5, 20, 100 mW
• Client-server distance: 10 meter (same floor)
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Measurement at Network Layer
General observation:
If transmit power<10 mW:
• MAC-layer retransmits
• rightward skew
• unacceptable perf.
If transmit power20 mW:
• acceptable performance
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Experiment 6: Wireless Channel Characteristics
Wireless Internet is characterized by limited
bandwidth, high error rates, and interference.
Purpose: to study the impact of the wireless channel
characteristics on wireless Web performance
Design:
• Number of Clients: 1
• HTTP transaction rate: 10 req/sec (fixed)
• HTTP transfer size: 1 KB (fixed)
• Persistent connection: no
• Transmit power: 100 mW
• Client-server distance: 1m, 10m, 100m
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Measurement at Network Layer (100m scenario)
Low load: 10 req/sec
Significant skew to the
tail of the distribution,
Some periodicity (why?)
Medium load: 50 req/sec
Significant skew to the
tail of the distribution
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6. Summary and Conclusions
 What we did: wireless Web server, portable nw
• Application-layer measurements (httperf)
• Network-layer measurements (Wireless Sniffer)
 Our results show:
• Server capability: 100 conn/sec for non-persistent
HTTP with throughputs up to 4 Mbps (adequate?)
• Bottleneck: at wireless network interface
• Some “network thrashing” for large HTTP transfers
when the network utilization is high (aborts, resets)
• Effect of wireless channel on performance at
TCP and HTTP-level (MAC-layer retransmits)
• Power consumption issue for mobile client and server
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7. Future Work
 Explaining the anomalies (fairness, periodicity)
 Better system instrumentation (Linux)
 More realistic Web workloads
 Larger WLAN testing (classroom scenario)
 Repeat experiments with IEEE 802.11a (55 Mbps)
 Kenny’s M.Sc. Thesis...
 Another paper?
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